1. Field of the Invention
The present invention relates to an antenna preferably for use in a wireless communication system such as an RFID (Radio Frequency Identification) system, and an RFID device including the antenna, and, in particular, relates to an antenna and an RFID device, applied to an RFID system of an HF band.
2. Description of the Related Art
In recent years, as a wireless communication system for performing information management of articles, an RFID system has been put to practical use, the RFID system establishing communication between a reader/writer generating an induction magnetic field and an RFID tag attached to an article on the basis of a non-contact method utilizing an electromagnetic field, and transmitting predetermined information. Here, the RFID tag includes an RFIC chip storing therein predetermined information and processing a predetermined RF signal and an antenna transmitting and receiving the RF signal.
For example, in Japanese Unexamined Patent Application Publication No. 2002-042083, an RFID tag utilizing a booster coil is disclosed. FIG. 1 is a plan view illustrating the arrangement of the booster coil and an IC device, included in the RFID tag. This RFID tag includes an RFIC 2 in which an antenna coil is integrally formed, an insulating member 6 in which a booster coil 3 and conductor films 4a and 4b used for electrostatic capacitance connection are provided, and a substrate integrally encasing these elements. In the RFIC 2, a rectangular spiral-shaped antenna coil is integrally formed, and the antenna coil is mounted so as to face the booster coil forming surface side of the insulating member 6.
On the back surface of the insulating member 6, conductor films 5a and 5b, which are used for electrostatic capacitance connection and face the conductor films 4a and 4b, are provided. In addition, as described above, the conductor films 4a and 4b, which are used for electrostatic capacitance connection and provided on the front surface side of the insulating member 6, are electrically connected through the booster coil 3, and the conductor films, which are used for electrostatic capacitance connection and formed on the back surface side of the insulating member 6, are electrically connected through a conductive wire.
In this RFID tag, the antenna coil of the RFIC 2 and the booster coil 3 are electromagnetic-field-coupled to each other, and a signal is transmitted between the RFIC 2 and the booster coil 3.
However, since, in such an RFID tag as illustrated in FIG. 1, the antenna coil has the same size as that of the RFIC chip and the booster coil has a card size, the sizes of both coils are significantly different from each other. Therefore, it is difficult to enhance the degree of coupling between the antenna coil and the booster coil. In addition, while, in Japanese Unexamined Patent Application Publication No. 2002-042083, a structure is disclosed in which the shape of a portion that is included in the booster coil and in which the RFIC chip is mounted, is turned into a shape closely related to the antenna coil, thereby enhancing the degree of coupling between the antenna coil on an RFIC chip side and the booster coil, the shape of the booster coil tends to become complex and the outside dimension of the booster coil tends to become large, in this structure.
In addition, in the antenna including the antenna coil and the booster coil, usually there occurs a situation where magnetic fluxes passing through a region in which the antenna coil and the booster coil overlap with each other or the vicinity of the region, cancel each other out. Also in the antenna illustrated in FIG. 1, for example, while each of magnetic fluxes B0 and B1 passes through the antenna coil and the booster coil in a same direction, a magnetic flux B2 passes through the antenna coil and the booster coil in an opposite direction. Therefore, in some cases, there occurs a null point at which a magnetic field formed owing to the antenna coil and a magnetic field formed owing to the booster coil cancel each other out. At this null point, it is hard to perform reading and writing.